1. Field of the Invention
The present invention relates to a toner to be used in electrophotographic apparatuses such as copiers, electrostatic printing appratuses, printers, facsimiles, and electrostatic recording apparatuses, also relates to an image forming apparatuse, an image forming method, and a process cartridge.
2. Description of the Related Art
Conventionally, there have been known various image forming methods by electrophotographic process. Generally, a latent electrostatic image is formed by charging a surface of a latent electrostatic image bearing member (otherwise referred to as “photoconductor”, “electrophotographic photoconductor”, or “image bearing member”) and exposing the charged surface of the latent electrostatic image bearing member. Subsequently, the latent electrostatic image is developed using a toner to form a visible image on the latent electrostatic image bearing member. The visible image is directly transferred onto a recording medium or transferred thereonto via an intermediate transfer member, and the transferred image is heated and/or pressurized, thereby a recorded matter with an image formed on the recording medium can be obtained. A residual toner remaining on the latent electrostatic image bearing member that has gone through the transfer of the visible image is removed by a known method, such as the use of a blade, a brush, a roller or the like.
Generally, as full-color image forming apparatuses utilizing such an electrophotographic process, there are known two types of process. One electrophotographic process is called a single-electrophotographic process (or single drum method), in which one latent electrostatic bearing member is mounted in an image forming apparatus, and four-developing units corresponding to four colors of cyan, magenta, yellow and black are mounted. In such a single-electrophotographic process, a four-color visible image is formed on a latent electrostatic image bearing member or a recording medium. The single-process makes it possible to share the same units such as a charging unit, an exposing unit, a transfer unit and a cleaning unit which are disposed around a latent electrostatic image bearing member, and allows for a more compact and lower-cost design than the after-mentioned tandem process.
Meanwhile, the other electrophotographic process is called a tandem process (or tandem drum method) in which a plurality of latent electrostatic image bearing members is mounted in an image forming apparatus (see Japanese Patent Application Laid-Open (JP-A) No. 5-341617). Typically, a charging unit, a developing unit, a transfer unit, and a cleaning unit are disposed for each latent electrostatic image bearing member, they integrally form one image forming element, and a plural number of the image forming elements (typically four image forming elements) are loaded. In the tandem process, a one-color visible image is formed by one image forming element, four-color visible images are sequentially transferred onto a recording medium to thereby form a full-color image. The tandem process allows for forming visible images for each color by parallel processing, and thus it enables high-speed image formation. More specifically, the tandem process requires about only one-fourth the time required for image formation in the above-mentioned single electrophotographic process and enables high-speed printing four times as high as that of the single electrophotographic process. Further, the tandem process makes it possible to substantially enhance the durability of each unit in image forming elements, including latent electrostatic image bearing members. This is because, in the single electrophotographic process, one latent electrophotographic image bearing member is subjected to four times of each of the steps of charging, exposing, developing and transferring to form one full-color image. In contrast to the single electrophotographic process, the tandem process requires only one-time operation of the above steps with the use of one latent electrostatic image bearing member.
However, the tandem process has a disadvantage in that it requires arrangement of a plurality of image forming elements, and it results in large size of the entire image forming apparatus and high-costs.
To solve the problem, the following measures have been taken. Namely, a latent electrostatic image bearing member itself is made to have a smaller diameter, and the size of each of units disposed around the latent electrostatic image bearing member is reduced, thereby making one image forming element smaller. As a result, it achieves an effect on not only reduction in size of image forming apparatuses but also reduction in material costs, and the total reduction in cost has been achieved. However, with improved compactness and downsizing of image forming apparatuses, new problems have arisen, that is, each of units loaded in image forming elements needs to achieve high performance and drastically improved stability.
Further, recently, with increased requirements for energy conservation and higher speed processing of image forming apparatuses such as printers, copiers, and facsimiles, market demands for energy conservation and high-speed performance of these apparatuses have more increased. In order to achieve such performance, it is important to improve thermal efficiency of fixing units in image forming apparatuses.
Generally, in image forming apparatuses, an unfixed toner image is formed on a recording medium such as recording sheet, printing paper, photosensitive paper and electrostatic recording paper through the use of an indirect transfer process or a direct transfer method, according to an image forming process such as electrophotographic recording, electrostatic recording and magnetic recording. As fixing units for fixing such unfixed toner images, contact heating methods such as heat roller method, film heating method, electromagnetic induction heating method are widely used.
The heat roller type fixing units have inside thereof a heat source such as halogen lamp and have a basic structure of a pair of rotational rollers composed of a fixing roller whose temperature is adjusted to a predetermined temperature, and a pressurizing roller that is pressure contact with the fixing roller. A recording medium is inserted into a contact portion of the pair of rotational rollers (a so-called nip portion), conveyed, and an unfixed toner image is melted by applying heat and pressure to the unfixed toner image from the fixing roller and the pressurizing roller so as to be fixed on the recording medium.
Film heating type fixing units are proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 63-313182 and 1-263679. In the film heating type fixing units, a recording medium is closely contacted with a heater which is stationarily supported by a support member via a thin fixing film having heat resistance, the fixing film is slid over the heater while being moved, and heat from the heater is supplied to the recording medium via the fixing film.
For the heater, a ceramic heater provided with a resistive layer on a ceramic substrate such as alumina and aluminum nitride which have physical properties of heat resistance, insulation properties, thermal conductivity, and the like. The use of such a fixing unit makes it possible to use a thin film with low heat volume as a fixing film and shorten the warm-up time, and enables quick-start and energy saving.
For the fixing unit of electromagnetic induction heating type, a technique is proposed in which Joule heat is generated due to the eddy current generated at a magnetic metal member through AC magnetic filed to thereby induce heat in a heater including the metal member (see Japanese Patent Application Laid-Open (JP-A) No. 8-22206).
In such a fixing unit of electromagnetic induction heating type, a film having a rubber-made elastic layer is placed between a heater and a recording medium in order to uniformly heat and melt a visible image in a fully wrapped state. When the rubber-made elastic layer is made of silicone rubber or the like, the heat responsiveness degrades due to its low thermal conductivity, and the temperature between the inside surface and the outside surface of a film which makes contact with toner significantly differs. When there exist a large amount of toner adhesion, the surface temperature of a belt surface rapidly decreases, which renders the fixing property to be ensured sufficiently, resulting in so-called cold offset.
Further, in the fixing step, a toner is required to have releasing property (otherwise referred to as “offset resistance” to a heating member. Such offset resistance can be improved by making a releasing agent exist on a surface of toner, however, when a not-specified toner is reused, not only the amount of a releasing agent residing on the toner surface may be reduced but also particles designed so as to have a core-shell structure with two or more layers may eccentrically exist on the toner surface, thereby the lower limit fixing temperature is inconveniently raised, causing insufficient low-temperature fixing property, i.e. energy saving fixing property. In a low-temperature fixing system that is required to provide further lower-temperature fixing property, a fixing defect occurs due to inorganic fine particles which eccentrically exist on the toner surface, and the fixing temperature width cannot be ensured.
While a toner excellent in low-temperature fixing property is requested, a toner having conflicting properties such as offset resistance and storage stability (blocking resistance) is requested. As a toner satisfying these requirements, toners are proposed which contain a polyester resin using rosins as acid component (see Japanese Patent Application Laid-Open (JP-A) Nos. 4-70765 and 2007-139813).
In the meanwhile, it is reported that as to a so-called capsulated toner composed of a core material and shells, chargeability and developing ability of the toner are improved by surface-treating a colorant in the core material with rosins such as natural rosins and abietic acid derivatives (see Japanese Patent Application Laid-Open (JP-A) No. 8-54755). It is also reported that a color toner containing a colorant that has been surface-treated with abietic acid is excellent in transferability, cleanability, and charge stability, and with the use of the toner makes it possible to obtain favorable full-color images (see Japanese Patent Application Laid-Open (JP-A) No. 7-128911).
In these techniques, rosin derivatives such as abietic acid having a heterocyclic aromatic ring are added to colorants. It is true that the dispersibility of the colorant is improved by adding these rosin derivatives, and transferability and charge stability are favorably exhibited in the early stage of use of the toner. However, these rosin derivatives are low-molecular weight materials and are not chemically bonded to binder resins. Therefore, a toner containing a large amount of a rosin derivative as a low-molecular weight material is likely to adhere onto a carrier in the case of a two-component developer and adhere onto a charge roller in the case of a one-component developer, and it is difficult to maintain the transferability and charge stability that have been obtained in the early stage of use. Further, in recent years, as the developing speed is increased, higher charge build-up property is requested. However, these toners do not have a sufficient charge build-up property.
Accordingly, it is desired to promptly provide an image forming apparatus, an image forming method and a process cartridge which are excellent in chargeability, anti-spent property (carrier-spent resistance), and transferability and are capable of forming high-quality images without causing a substantial degradation in its quality for a long period of time.
Meanwhile, in order to improve the charge build-up property of toner, the most common method is to add a charge controlling agent into a toner. Examples of typical charge controlling agents are chelate compounds with salicylic acid, oxysalicylic acid or the like as ligands. Metal complex salts of these salicylic acid derivatives are proposed in Japanese Patent Application Laid-Open (JP-A) No. 62-145255 and in paragraph 6 of Japanese Patent Application Publication (JP-B) No. 55-42752, for example. However, in these proposals, it is unfavorable in terms of environmental protection because the toners contain heavy metal such as Cr and Co. Then, in view of environmental protection, a number of salicylic acid derivatives that primarily contain Fe as a center metal and contain no heavy metals of Cr, Co, etc. are proposed. For example, effect of using a metal complex of salicylic acid derivatives having a carboxyl group or sulfoxyl group as a substituent is disclosed (see Japanese Patent Application Laid-Open (JP-A) No. 1-309072), and effect of using a combination of a specific resin and an iron complex containing salicylic acid as ligands is disclosed (see Japanese Patent Application Laid-Open (JP-A) Nos. 9-325520, 7-230188 and 2001-343787).
However, in these proposals, the effect is exhibited only when limited to the use of a combination of a specific resin and a metal complex of salicylic acid derivative. Further, in order to obtain uniformity of charge amount and stability among toner materials, it is important to uniformly and finely disperse any of these metal complexes of salicylic acid derivative in toner. However, it is difficult to sufficiently finely disperse these metal complexes in toner with the use of conventional toner binder resins, and there have been a such a problem with frequent occurrence of abnormal images such as toner fogging.
Accordingly, it is desired to promptly provide a toner capable of maintaining its excellent charge build-up property, charge stability and transferability without substantially causing smear onto carrier and charge rollers for a long period of time and to provide a developer using the toner and an image forming apparatus using the toner, each of which allows for forming extremely high-quality images without substantially causing change in color tone for a long period of time, and without substantially causing abnormal images such as a reduction in image density, and fogging.
Further, in a fixing step based on contact heating process, in which fixing is carried out using a heating member such as heat roller, toner is required to have releasing property (hereinafter, may be referred to as “offset resistance”) to such a heating member. The offset resistance can be improved by adding a releasing agent into toner, and the state of toner residing inside the toner significantly affects the releasing property. It is desired that a releasing agent be uniformly dispersed in the toner, however, when excessively finely dispersed, the releasing agent is least likely to exude on the toner, and thus it is preferred that the releasing agent have an appropriate dispersion diameter. Polyester resins using aliphatic alcohol are highly compatible with commonly used releasing agents and have poor releasing property. However, polyester resins make it possible to set molecular weight high because of its rigidity of the structure, if the fixing is carried out at a same temperature. The polyester resins have more excellent heat-resistance/storage stability than styrene-acrylic resins. As to styrene-acrylic resins, when imparting fusibility at low temperature, it is necessary to set its molecular weight low. As a result, styrene-acrylic resins inconveniently become brittle and the heat-resistance/storage stability degrades, although these resins have advantages in that they are inexpensive, and the compatibility with releasing agents is readily controlled because of their degrees of design freedom, such as providing with a functional group.
For example, a number of methods of mixing and using a styrene-acrylic resin with a polyester resin have been proposed so far (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2003-255611).
Further, a method of using a crystalline polyester resin and a styrene-acrylic resin is proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2006-171364).
However, in any of these proposals, depending on the compatibility between a styrene-acrylic resin and a polyester resin, smear of the resin component onto carrier and a change in physical properties are liable to occur in accordance with separation of the resin in the toner.
Further, use of a copolymer between a styrene-acrylic resin and a polyester resin is also proposed, however, it is necessary to introduce the styrene-acrylic resin into the polyester resin within the range where the low-temperature fixing property is not impaired, and it is problematic because the heat-resistance/storage stability is sometimes insufficient, and sufficient low-temperature fixing property cannot be obtained.
Furthermore, an example of using a crystalline polyester resin and a styrene-acrylic resin in combination is also proposed with expectation of improvement in low-temperature fixing property. However, the proposal has a problem with separation among resins and separation of pigment, etc. because of the crystallinity of the polyester resin.
To improve the compatibility between a styrene-acrylic resin and a polyester resin, methods of using a block polymer or a copolymer of polyester resin with a styrene-acrylic resin are proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2005-266400).
Further, as a method of introducing styrene-acrylic resin components at the time of granulation of toner, utilization of a chemical toner production method is proposed (see Japanese Patent Application Laid-Open (JP-A) No. 2001-265058 and Japanese Patent (JP-B) No. 3203451). However, according to the proposal, when a styrene-acrylic resin component is introduced to a polyester resin while maintaining fixing property of the polyester resin, it causes a problem with brittleness and heat-resistance/storage stability of styrene-acrylic resin sites.
Furthermore, a toner is proposed which contains a nonlinear crosslinked polyester resin using a rosin or rosins as acidic component(s) in a polyester resin (see Japanese Patent Application Laid-Open (JP-A) No. 4-70765). According to the proposal, an aliphatic alcohol, an aromatic alcohol, an unsaturated dicarboxylic acid having 4 to 10 carbon atoms, a terephthalic acid, and other carboxylic acids such as trimellitic acid are compounded in addition to rosin(s). However, a problem with odor specific to rosins arises when heated, and the rosin components which have low-molecular weight material are likely to adhere onto members such as charge rollers.
Thus, a toner which is provided with excellent low-temperature fixing property and excellent heat-resistance/storage stability and does not substantially cause smear of the toner and releasing agent onto a carrier and other members even in long-time use and related techniques have not yet been provided so far, and the current circumstance is that further improvements and developments of toner are desired.